I found the PPL books quite easy to understand, and only required me to go through it without major problems, and when it came to the exams, the question asked, by looking at the choices of exams it sort of jogged your memory, which made it easy to select the correct answer.
Now ATPL JAA, well that as mentioned above requires a high understanding, because even when you are asked a question it is so ambiguous, you think to yourself, "What the hell are they asking", the turn the most simple question into something that appears really hard, and to make matters worst, the answers they give you can really confuse, they also might give a statement and just change one word to catch you out, for example a statement may mention "AND" and the other mentions "OR" because you are under pressure, you can easily go off track.
It sad, but true that sometimes these subjects don't try to test your understanding but rather to catch you out.
For example in Performace or Flight Planning, reading off those horrible unclear graphs you could come out with an accurate answer of lets say for eg: 12.5M
Answers available are;
A) 12.3M
B) 12.6M
C) 12.4M
D) 13.0M
Now your answer appears in between 12.4 and 12.6M, the question is what is the correct answer.
Also be aware of the units aswell, as that can sometimes catch you out.
I don't mean to sound negative, but just to make you try and appreciate the jump between PPL to ATPL.
Good Luck.